Balun

ABSTRACT

A balun in which the phase shift may be reduced significantly is disclosed. The balun has three lines, i.e. a first line b, a second line a and a third line c, arranged in parallel with the ground surface. The second line a and the third line c are arranged at the same height from the ground surface GC, the longitudinal length of each respective one of the first line b, second line a and third line c are specified to be equal to a quarter (¼) of the wavelength at the central frequency in the working band, and the capacitance Ca between the second line a and the ground surface GC is specified to be equal to the capacitance Cab between the second line a and the first line b. Furthermore, the distance h 3  between the center of each respective one of the second line a and third line c in the height direction and the ground surface GC located closer to each respective one of the second line a and third line c is specified to be longer than the distance h 2  between the center of the first line b in the height direction and the center of each respective one of the second line a and third line c in the height direction, or the permittivity of a dielectric D 3  is specified to be less than that of a dielectric D 2.

TECHNICAL FIELD

The present invention relates to a balun having three lines arranged inparallel with the ground surface.

BACKGROUND

For the high-speed LSI for the recent wireless LAN, such as Bluetooth,the balanced-mode signals are in most cases produced in order toincrease the signal noise margin.

As the wireless circuit generally employs an unbalanced circuit,however, the balun (balanced unbalanced conversion circuit) constitutesan essential element for performing this conversion.

The balun may comprise two branch circuits, that is, a ¼-wavelength lineand a ¾-wavelength line.

When the balun includes the ¼-wavelength line and ¾-wavelegth line,however, they have a different line length. Thus, even if a frequencycan have a phase of 180 degrees with regard to the central frequency,the phase shift may be increased if the frequency is shifted within aband.

In view of the above fact, it is an object of the present invention toprovide a balun in which the phase shift can be reduced significantly.

SUMMARY OF THE INVENTION

In order to solve the problems described above, the inventors of thepresent invention have studied the problems extensively and earnestly,and have discovered that the problems can be solved by providing abalanced circuit that includes three lines each having the line lengthequal to ¼of the wavelength at the central frequency in the workingband. The present invention is thus based on the above discovery.

Specifically, the balun of the present invention is one that has threelines, that is, a first line, a second line and a third line, arrangedin parallel with the ground surface, wherein the second line and thethird line are arranged at the same height from the ground surface, thelongitudinal length of each respective one of the first line, secondline and third line is specified to a quarter of the central frequencyin the working band, and the capacitance between the second line and theground surface is specified to be equal to the capacitance between thesecond line and the first line.

According to the present invention, the distance between the center ofeach respective one of the second line and third line in the directionof the height and the ground surface located closer to the second lineand third line is specified to be larger than the distance between thecenter of first line in the direction of the height and the center ofeach respective one of the second line and third line in the directionof the height. As one alternative, the permittivity of a dielectricbetween the plane formed by the center of each respective one of thesecond line and third line in the direction of the height may bespecified to be less than the permittivity of a dialectic between theplane formed by the center of the first line in the direction of theheight and the plane formed by the center of each respective one of thesecond line and third line in the direction of the height.

As a further alternative, the length of each respective one of thesecond line and third line in the direction of the width may bespecified to be equal, the second line and third line may be arrangedsymmetrically with regard to the plane formed by the center of the firstline in the direction of the width, one terminal of the first line beingassumed as an input terminal for unbalanced-mode signals may beconnected to one terminal of the third line as such input terminal, theother terminal of the first line and one terminal of the second line maybe connected to the ground surface, respectively, and the other terminalof the second line and the other terminal of the third line may beassumed as output terminals for balanced-mode signals, with theimpedance of the input terminal for unbalanced-mode signals and theimpedance of the output terminal for balanced-mode signals beingspecified to satisfy the following relationship defined below:(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in)×Z_(out))^(1/2)where Ca is the capacitance (C) between the second line and the groundsurface, Cac is the capacitance (C) between the second line and thethird line, ε₀ is the permittivity in the vacuum, ε_(r) is the specificpermittivity, Z_(air) is the characteristic impedance (Ω) in the vacuum,Z_(in) is the impedance (Ω) of the input terminal for unbalanced-modesignals, and Z_(out) is the impedance (Ω) of the output terminal forbalanced-mode signals.

FIG. 1 is a sectional view illustrating one embodiment of the balunaccording to the present invention, and FIG. 2 is a top viewillustrating one embodiment of the balun according to the presentinvention.

According to the present invention, the longitudinal length of eachrespective one of the first length a, second length a and third length cis specified to a quarter (¼) of the wavelength of the central frequencyin the working band, the second line a and third line c are arranged atthe same height from the ground surface GC, and the capacitance Cabetween the second line a and ground surface GC is set to be equal tothe capacitance Cab between the second line a and first line b(hereinafter, this equality will sometimes be referred to as “Ca=Cab”).

In the balun of the present invention, it is preferred that the distancebetween the center of each respective one of the second line a and thirdline c in the direction of the height and the ground surface GC locatedcloser to the second line a and third line c (which will sometimes bereferred to as “h3”) should be specified to be longer than the distancebetween the first line b in the direction of the height and the distanceof each respective one of the second line a and third line c in thedirection of the height (which will sometimes be referred to as “h2”).In the following description, it should be noted that the distancebetween the center of the first line b in the direction of the heightand the ground surface GC located closer to the first line b willsometimes be referred to as “h1”.

In the balun of the present invention, furthermore, it is preferred thatinstead of being h2 <h3, the permittivity ε 3 of a dialectic (which willsometimes be referred to as “D3”) between the plane formed by the centerof each respective one of the second line a and third line c in thedirection of the height and the ground surface GC located closer to thesecond line a and third line c should be specified to be less than thepermittivity ε 2 of a dialectic (which will sometimes be referred to as“D2”) between the plane formed by the center of the first line b in thedirection of the height and the plane formed by the center of eachrespective one of the second line a and third line c in the direction ofthe height (which will sometimes be referred to as “ε3<ε2”). In thefollowing description, it should be noted that the dialectic between theplane formed by the center of the first line b and the ground surfacelocated closer to the first line b will sometimes be referred to as“D1”.

In the balun of the present invention in which h2<h3 is given, thedielectric having the relative permittivity ε_(r) is included so thatthe permittivity ε_(r) is equal for all of the dielectric 1, dielectric2 and dielectric 3.

The following describes the results of the electromagnetic fieldanalysis that was performed using the device simulator for thecapacitance Ca between the second line a and ground surface GC and thecapacitance Cab between the second line a and first line b.

Initially, for h2=h3=2 micrometers being given, the capacitance Ca andthe capacitance Cab have been examined to determine how thosecapacitances will change as the interval Sac between the second line aand third line c is varied. The results are given in FIG. 3.

It may be seen from FIG. 3 that Ca>Cab is satisfied for all of theintervals Sac that have been varied, but Ca=Cab is not satisfied.

Accordingly, it may be appreciated that at least Ca=Cab can be satisfiedif any of the following conditions is satisfied.

-   a) ha<h3

If this condition is satisfied, the incremental amount of thecapacitance Ca will become greater than that of the capacitance Cab ifthe interval Sac is increased. If Ca<Cab is true when the interval Sacis initially small, enlarging the interval Sac gradually will cause Caversus Cab to change like Ca<Cab, Ca=Cab, and Ca>Cab. Thus, the intervalSac that satisfies Ca=Cab can be estimated.

FIG. 4 shows how the capacitance Ca versus the capacitance Cab willchange if the interval Sac is changed when h2=1.5 micrometers and h3=2micrometers are given. It may be seen from FIG. 4 that Ca=Cab can besatisfied when the interval Sac is about 10.3 micrometers.

-   b) ε3 <ε2

If this condition is satisfied, the interval Sac that satisfies Cab=Cabis available even if h2=h3.

If ε3<ε2, the medium will become isotropic no longer in its strict senseof the word although it has little effect. The balun can thus beconstructed as is the case with h2<h3.

In the present invention, furthermore, it is preferred that the lengthof the second line a in the direction of the width (which will sometimesbe referred to as “Wa”) should be specified to be equal to the length ofthe third line c in the direction of the width and shorter than thelength of the first line b in the direction of the width (which willsometimes be referred to as “Wb”). It is also preferred that thethickness of each respective one of the first line b, second line a andthird line c (which will sometimes be referred to as “t”) should beequally the same.

It is preferred that the second line a and third line c should bearranged symmetrically with regard to the line formed by the center ofthe first line b in the direction of the width and its extension.Preferably, one terminal of the first line b should be assumed as theinput terminal for unbalanced-mode signals which is connected to oneterminal of the third line c, the other terminal of the first line b andone terminal of the second line a should be connected to the groundsurface GC, respectively, and the other terminal of the second line aand the other terminal of the third line c should be assumed as theoutput terminal for balanced-mode signals.

It is preferred that the length of each respective one of the secondline a and third line c in the direction of the width (Wa) and theinterval between the second line a and third line c in the direction ofthe width (which will sometimes be referred to as “Sac”) should bechosen as appropriate in order to satisfy the following relationship aswell as the condition specified by Ca=Cab.(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in) ×Z _(out))^(1/2)where Ca is the capacitance (C) between the second line a and the groundsurface GC, Cac is the capacitance (C) between the second line a and thethird line c, ε₀ is the permittivity in the vacuum, ε_(r) is therelative permittivity, Z_(air) is the characteristic impedance (Ω) inthe vacuum, Z_(in) is the impedance (Ω) of the input terminal forunbalanced-mode signals, and Z_(out) is the impedance (Ω) of the outputterminal for balanced-mode signals.

The following explains the process of deriving the above relationship,in which it is assumed that there is no loss since the line conductorloss and the dielectric loss are usually negligible.

The Y matrix (6 rows×6 columns) for the three-line balun shown in FIG. 2in which the line length is equal to a quarter of the wavelength at thecentral frequency in the working band may be given as follows:

$Y = {1/{{ku}\begin{pmatrix}{{\omega\; C},} & {{- \omega}\;{C\left( {1 - u^{2}} \right)}^{1/2}} \\{{{- \omega}\;{C\left( {1 - u^{2}} \right)}^{1/2}},} & {\omega\; C}\end{pmatrix}}}$where, ω refers to the frequency and u=j×tan (KL) (K is the phaseconstant in the dielectric and L is the line length).

C is the C matrix for the 3 lines, that is,

$C = \begin{pmatrix}{{{Ca} + {Cab} + {Cac}},} & {{- {Cab}},} & {- {Cac}} \\{{- {Cab}},} & {{{Cb} + {2{Cab}}},} & {- {Cab}} \\{{- {Cac}},} & {{- {Cab}},} & {{Ca} + {Cab} + {Cac}}\end{pmatrix}$where, Cb refers to the capacitance (C) between the first line andground surface.

As the line length L is specified to be equal to a quarter of thewavelength at the central frequency in the working band, 1/u canapproximate to zero (0), and (1−u²)^(1/2)/u can approximate to −j.

Accordingly, the Y matrix can be arranged as follows:

$Y = {j\;{\omega/k}\;\begin{pmatrix}{0,} & C \\{C,} & 0\end{pmatrix}}$ $Y = {j\;{\omega/k_{z}}\;\begin{pmatrix}{0,} & {0,} & {0,} & {{{Ca} + {Cab} + {Cac}},} & {{- {Cab}},} & {- {Cac}} \\{0,} & {0,} & {0,} & {{- {Cab}},} & {{{Cb} + {2{Cab}}},} & {- {Cab}} \\{0,} & {0,} & {0,} & {{- {Cac}},} & {{- {Cab}},} & {{Ca} + {Cab} + {Cac}} \\{{{Ca} + {Cab} + {Cac}},} & {{- {Cab}},} & {{Cac},} & {0,} & {0,} & 0 \\{{- {Cab}},} & {{{Cb} + {2{Cab}}},} & {{- {Cab}},} & {0,} & {0,} & 0 \\{{- {Cac}},} & {{- {Cab}},} & {{{Ca} + {Cab} + {Cac}},} & {0,} & {0,} & 0\end{pmatrix}}$

In FIG. 2 and in the following description, it is assumed that the sixterminals are designated as u terminal, v terminal, w terminal, xterminal, y terminal and z terminal, respectively, and the inputterminal 1 (P_(in) 1) is electrically connected to the v terminal of thefirst line and to the w terminal of the third line, and the y terminalof the first line and the u terminal of the second line are connected tothe ground surface, respectively, with the x terminal of the second linebeing electrically connected to the output terminal (P_(out) 2) and thez terminal of the third line being electrically connected to the outputterminal 3 (P_(out) 3).

Under the above assumption, the following equation will hold true:Vu (voltage at u terminal)=Vy (voltage at y terminal)=0Vv (voltage at v terminal)=Vw (voltage at w terminal)=V1 (voltage atinput terminal 1J1 (current through input terminal 1)=Jv (current through v terminal)+Jw(current through w terminal)Vx (voltage at x terminal)=V2 (voltage at output terminal 2)Jx (current through x terminal)=V2 (voltage at output terminal 2)Vz (voltage at z terminal)=V3 (voltage at output terminal 3)Jz (current through z terminal)=J3 (current through output terminal 3)Accordingly, the following equation will hold true:

$\begin{pmatrix}{Ju} \\{Jv} \\{Jw} \\{J2} \\{Jy} \\{J3}\end{pmatrix} = {j\;{\omega/k_{z}}\;\begin{pmatrix}{0,} & {0,} & {0,} & {{{Ca} + {Cab} + {Cac}},} & {{- {Cab}},} & {- {Cac}} \\{0,} & {0,} & {0,} & {{- {Cab}},} & {{{Cb} + {2{Cab}}},} & {- {Cab}} \\{0,} & {0,} & {0,} & {{- {Cac}},} & {{- {Cab}},} & {{Ca} + {Cab} + {Cac}} \\{{{Ca} + {Cab} + {Cac}},} & {{- {Cab}},} & {{Cac},} & {0,} & {0,} & 0 \\{{- {Cab}},} & {{{Cb} + {2{Cab}}},} & {{- {Cab}},} & {0,} & {0,} & 0 \\{{- {Cac}},} & {{- {Cab}},} & {{{Ca} + {Cab} + {Cac}},} & {0,} & {0,} & 0\end{pmatrix}}$ $\begin{pmatrix}{Ju} \\{Jv} \\{Jw} \\{J2} \\{Jy} \\{J3}\end{pmatrix} = {j\;{\omega/k_{z}}\;\begin{pmatrix}{{\left( {{Ca} + {Cab} + {Cac}} \right)\;{V2}} - {CacV3}} \\{{- {CabV2}} - {CabV3}} \\{{- {CacV2}} + {\left( {{Ca} + {Cab} + {Cac}} \right){V3}}} \\{{- \left( {{Cab} + {Cac}} \right)}{V1}} \\{\left( {{Cb} + {Cab}} \right){V1}} \\{\left( {{Ca} + {Cac}} \right){V1}}\end{pmatrix}}$J1 (current through input terminal 1) has the following relationship:J1=Jv+JwJ1=jω{−Cab×V2−Cab×V3−Cac×V2+(Ca+Cab+Cac) V3}/k _(z)J1=jω{−(Cab+Cac)×V2+(Ca+Cac) V3}/k _(z)

Accordingly, the Y matrix for the three terminals may be given in termsof the following equation:

$Y = {j\;{\omega/k_{z}}\;\begin{pmatrix}{0,} & {{- \left( {{Cab} + {Cac}} \right)},} & {{Ca} + {Cac}} \\{{- \left( {{Cab} + {Cac}} \right)},} & {0,} & 0 \\{{{Ca} + {Cac}},} & {0,} & 0\end{pmatrix}}$

Now, consider the case where the above Y matrix satisfies the conditionsspecified by the balun. Then, the lossless balun can satisfy thefollowing condition, considering the central symmetry:S ₁₁=0S ₂₁ =−S ₃₁=2^(1/2)×exp(jα)/2S ₂₂ −S ₃₂ =S ₃₃ −S ₂₃=0As the balun is lossless, the following equation will hold true:|S ₂₁|² +|S ₂₂|^(1/2) +|S ₂₃|²=1S _(ij) =S _(ji) i, j=1, 2, 3Therefore, the S matrix may be given as follows:

$S = \begin{pmatrix}{0,} & {{2^{{- 1}/2} \times {\exp\left( {j\;\alpha} \right)}},} & {{- 2^{{- 1}/2}} \times {\exp\left( {j\;\alpha} \right)}} \\{{2^{{- 1}/2} \times {\exp\left( {j\;\alpha} \right)}},} & {S_{22},} & S_{22} \\{{{- 2^{{- 1}/2}} \times {\exp\left( {j\;\alpha} \right)}},} & {S_{22},} & S_{22}\end{pmatrix}$where, |S₂₂|²=¼, thus S₂₂=exp (jβ)/2.

$S = \begin{pmatrix}{0,} & {{2^{{- 1}/2} \times {\exp\left( {j\;\alpha} \right)}},} & {{- 2^{{- 1}/2}} \times {\exp\left( {j\;\alpha} \right)}} \\{{2^{{- 1}/2} \times {\exp\left( {j\;\alpha} \right)}},} & {{{1/2} \times {\exp\left( {j\;\beta} \right)}},} & {{1/2} \times {\exp\left( {j\;\beta} \right)}} \\{{{- 2^{{- 1}/2}} \times {\exp\left( {j\;\alpha} \right)}},} & {{{1/2} \times {\exp\left( {j\;\beta} \right)}},} & {{1/2} \times {\exp\left( {j\;\beta} \right)}}\end{pmatrix}$

The above equation assumes that all of the input and output terminalshave the reference impedance, and if the above S matrix is convertedinto the Y matrix with the input/output terminal impedance being Z_(in),Z_(out)/2, the resulting Y matrix Yb will be obtained as follows:

${Yb} = \begin{pmatrix}{{Yb}_{11},} & {{Yb}_{12},} & {Yb}_{12} \\{{Yb}_{12},} & {{Yb}_{22},} & {Yb}_{23} \\{{Yb}_{12},} & {{Yb}_{23},} & {Yb}_{22}\end{pmatrix}$where,Yb ₁₁=−(1+exp(2jα)/{Z _(in)(−1+exp(2jα))}Yb ₁₂=2exp(jα)/{Z _(in) ×Z _(out))^(1/2)(−1+exp(2jα))}Yb ₂₂=−2(1+exp(j(2α+β)))/{Z _(out)(−1+exp(2jα) (1+exp(jβ))}Yb₂₃=2(exp(j2α)+exp(jβ))/{Z_(out)(−1+exp(2jα))(1+exp(jβ))}The Y matrix for the three terminals may be expressed in terms of thefollowing equation:

$Y = {j\;{\omega/k_{z}}\;\begin{pmatrix}{0,} & {{- \left( {{Cab} + {Cac}} \right)},} & {{Ca} + {Cac}} \\{{- \left( {{Cab} + {Cac}} \right)},} & {0,} & 0 \\{{{Ca} + {Cac}},} & {0,} & 0\end{pmatrix}}$In order that the above equation will be equal to the Y matrix of theinput/output terminal impedance Z_(in), Z_(out)/2, it is required thatexp(2αj)=−1, exp(βj)=1 exist. For α=π/2, β=π, Yb may be expressed asfollows:

${Yb} = \begin{pmatrix}{0,} & {{{- j}/\left( {Z_{in} \times Z_{out}} \right)^{1/2}},} & {j/\left( {Z_{in} \times Z_{out}} \right)^{1/2}} \\{{{- j}/\left( {Z_{in} \times Z_{out}} \right)^{1/2}},} & {0,} & 0 \\{{j/\left( {Z_{in} \times Z_{out}} \right)^{1/2}},} & {0,} & 0\end{pmatrix}$

The balun can be obtained if the construction of the C matrix isdesigned so that it can satisfy the following equations by comparing Yand Yb:ω(Cab+Cac)=k _(z)/(Z _(in) ×Z _(out))^(1/2)ω(Ca+Cac)=k _(z)/(Z _(in) ×Z _(out))^(1/2)That is,ω(Cab+Cac)=ω(Ca+Cac)=k _(z)/(Z _(in) Z _(out))^(1/2)Accordingly, the required condition can be expressed by the twoequations given below:Ca=Cabvp(Ca+Cac)=1/(Z _(in) ×Z _(out))^(1/2)

As the relative permeability is approximately 1 for the ordinary metals,the phase velocity Vp may be expressed as follows, usingZ_(air)=(μ₀/ε₀)^(1/2)=120π.vp=1/(εμ)^(1/2)vp=1/(ε_(r)ε₀μ₀)^(1/2)vp=1/(ε₀ ×Z _(air)×ε_(r) ^(1/2))Accordingly,(Ca+Cac)=1/vp(Z _(in) ×Z _(out))^(1/2)(Ca+Cac)=ε₀ ×Z _(air)×ε_(r) ^(1/2)/(Z _(in) ×Z _(out))^(1/2)From the above,(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in) ×Z _(out))^(1/2)

According to the present invention, the balun that provides any desiredinput/output impedance can be constructed by choosing the appropriatelength of each respective one of the second line and third line in thedirection of the width and the appropriate interval Sac between thesecond line a and third line c, even if the particular input/outputimpedance is specified for the balun.

FIG. 5 presents the values of the interval Sac, capacitance (Ca+Cab),and input/output impedance (Z_(in)×Z_(out))^(1/2) that will satisfy theCa=Cab condition by varying the length Wa of each respective one of thesecond line a and third line c in the direction of the width. It shouldbe noted that the length Wb of the first line b in the direction of thewidth is fixed to 16 micrometers.

It may become apparent from FIG. 5 that the balun that provides anydesired input/output impedance can be constructed by choosing theappropriate length Wa in the direction of the width and the appropriateinterval Sac, even if the particular input/output impedance is specifiedfor the balun.

In the strip line-type balun according to the present invention, itshould be noted that the property will remain unchanged if the absolutedimensions are varied because the aspect ratio can be the same. Thus,the same property can be obtained even if the aspect ratio is increasedor decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating one example of the balunaccording to the present invention, in which a refers to a second line,b refers to a first line, c refers to a third line, GC refers to aground surface, Wa refers to the length of each respective one of thesecond line a and third line c in the direction of the width, Wb refersto the length of the first line b in the direction of the width, trefers to the thickness of the first line b, second line a and thirdline c in the direction of the height, Sac refers to the intervalbetween the second line a and third line c in the direction of thewidth, and Sab refers to the location of one end of the second line a inthe direction of the width when one end of the first line b is given asa reference; it should be noted that when Sab is positive, the one endof second line is situated outside the one end of the first line b, andwhen Sab is negative, the one end of the second line is situated insidethe one end of the first line; the second line a and third line c arelocated symmetrically with regard to the line AA′; generally, h meansthe distance with regard to the ground surface GC; specifically, h1refers to the distance between the center of the first line b in thedirection of the height and the ground surface GC located closer to thefirst line b, h2 refers to the distance between the center of the firstline b in the direction of the height and the center of each respectiveone of the second line a and third line c in the direction of theheight, and h3 refers to the distance between the center of eachrespective one of the second line a and third line c in the direction ofthe height and the ground surface GC closer to the second line a andthird line c; D1 refers to a dielectric 1, D2 refers to a dielectric 2and D3 refers to a dielectric 3;

FIG. 2 is a top view illustrating one example of the equivalent circuitdiagram for the balun according to the present invention, in whichP_(in)1 refers to an input terminal 1, P_(out)2 refers to an outputterminal 2, and P_(out)3 refers to an output terminal 3, with u, v, w,x, y, and z representing six terminals; it should be noted that thelongitudinal length of each respective one of the first line b, secondline a and third line c is specified to be equal to one quarter of thewavelength at the central frequency in the working band;

FIG. 3 is a graph diagram that shows how the capacitance will be changedas the distance between the second line and third line is varied whenh2=h3;

FIG. 4 is a graph diagram that shows how the capacitance will be changedas the distance between the second line and third line is varied whenh2<h3;

FIG. 5 is a graph diagram that shows how the values for the distanceSab, capacitance (Ca+Cac), and input/output impedance(Z_(in)×Z_(out))^(1/2) will be changed to satisfy the condition ofCa=Cab as the length of each respective one of the second line and thirdline in the direction of the width is varied;

FIG. 6 is a graph diagram that shows the transmission characteristic tothe output terminal 2 (S₂₁) and the transmission characteristic to theoutput terminal 3 (S₃₁);

FIG. 7 is a graph diagram that shows the phase difference between theoutput terminal 2 and output terminal 3 when a signal is providedthrough the input terminal 1; and

FIG. 8 is a graph diagram that shows the variation of the reflectionloss for the input terminal 1 (S₁₁) as well as the variation of thereflection that occurs when differential amplitude is provided to theoutput terminal 2 and output terminal 3.

BEST MODES OF EMBODYING THE INVENTION

Several preferred embodiments of the present invention are now describedbelow. It should be understood, however, that the present invention isnot restricted to those embodiments, which may be modified in numerousways without departing from the spirit and scope of the invention asdefined in the appended claims.

Embodiment 1

In the first embodiment being described here, the balun of the presentinvention employs the construction in which it has the band of 2.45 GHz,the input impedance of 50 Ω and the output impedance of 100 Ω. Then, thebalun was examined to check the performance.

Since (Z_(in)×Z_(out))^(1/2)=70.7 Ω is given, h2=1.5 micrometers, h3=2micrometers, Wa=3.35 micrometers, Sab=0.17 micrometers, Sac=8.96micrometers, and Wb=16 micrometers are provided.

The line has the length of about 15.5 mm that generally corresponds to¼wavelength for the band of 2.45 GHz and permittivity of 3.6.

It should be noted that as the line has the length of 100 micrometersfor the actual print circuit board, the above parameters may bemultiplied by a factor of 100 such that h2=150 micrometers, h3=200micrometers, Wa=335 micrometers, Sab=17 micrometers, Sac=896micrometers, and Wb=1600 micrometers.

The results that have been obtained by the electromagnetic fieldsimulation are shown in FIGS. 6 through 8.

FIG. 6 is a graph diagram that shows the transmission characteristic forthe balanced terminal or output terminal 2 (S₂₁) and the transmissioncharacteristic for the balanced terminal or output terminal 3 (S₃₁) whena signal is provided through the unbalanced terminal or input terminal1. It may be seen from FIG. 6 that the amplitude is almost equal for theband of 2.45 GHz.

FIG. 7 is a graph diagram that shows the phase difference between theoutput terminal 2 and output terminal 3 when a signal is providedthrough the input terminal 1. It may be seen from FIG. 7 that the phasethat can occur is of substantially 180 degrees and the phase shift canbe reduced significantly.

FIG. 8 shows the reflection factor for the input terminal 1 (S₁₁) aswell as the reflection that occurs when differential amplitude isprovided at the output terminal 2 and output terminal 3((S₂₂+S₂₃−2×S₂₃)/2^(3/2), where S₂₂ represents the reflection factor forthe output terminal 2 and S₂₃ represents the transmission factor fromthe output terminal 3 to the output terminal 2). It may be seen fromFIG. 8 that less than 25 dB is provided at 2.45 GHz and the desiredoutput impedance can be obtained.

POSSIBLE INDUSTRIAL UTILITIES

It may be appreciated from the foregoing description that the balunaccording to the present invention provides the performance that isequivalent to the commercially available chip components, and permitsthe phase shift to be reduced significantly. Thus, the balun can beincorporated in the multi-layer circuit board, and can flexibly meet therequirements for producing many kinds of components as well as theshort-term production requirements. The balun can also meet therequirements for producing the drastically reduced size components.

1. A balun having three lines, including a first line, a second line anda third line, arranged in parallel with the ground surface, wherein saidsecond line and said third line are arranged at the same height from theground surface, the longitudinal length of each respective one of saidfirst line, said second line and said third line is specified to beequal to a quarter of the wavelength at the central frequency in theworking band, and the capacitance between said second line and theground surface is specified to be equal to the capacitance between saidsecond line and said first line.
 2. The balun as defined in claim 1,wherein the distance between the center of each respective one of saidsecond line and said third line in the direction of the height isspecified to be longer than the distance between the center of saidfirst line in the direction of the height and the center of eachrespective one of said second line and said third line in the directionof the height.
 3. The balun as defined in claim 1, wherein thepermittivity of a dielectric between the plane formed by the center ofeach respective one of said second line and said third line in thedirection of the height and the ground surface located closer to eachrespective one of said second line and said third line is specified tobe less than the permittivity of a dielectric between the plane formedby the center of said first line in the direction of the height and theplane formed by the center of each respective one of said second lineand said third line in the direction of the height.
 4. The balun asdefined in claim 1, wherein the length of each respective one of saidsecond line and said third line in the direction of the width isspecified to be equal, said second line and said third line are arrangedsymmetrically with regard to the plane formed by the center of saidfirst line in the direction of the width, one terminal of said firstline being assumed as an input terminal for unbalanced-mode signals isconnected to one terminal of said third line as said input terminal forunbalanced-mode signals, the other terminal of said first line and oneterminal of said second line are connected to the ground surfacerespectively, and the other terminal of said second line and the otherterminal of said third line are assumed as output terminals forbalanced-mode signal, the impedance of said input terminal forunbalanced-mode signals and the impedance of said each output terminalfor balance signals being specified to satisfy the followingrelationship defined below:(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in) ×Z _(out))^(1/2) where Ca isthe capacitance (C) between said second line and the ground surface, Cacis the capacitance (C) between said second line and said third line, ε₀is the permittivity in the vacuum, ε_(r) is the relative permittivity,Z_(air) is the characteristic impedance (Ω) in the vacuum, Z_(in) is theimpedance (Ω) of said input terminal for unbalanced-mode signals, andZ_(out) is the impedance (Ω) of said output terminal for balanced-modesignals.
 5. The balun as defined in claim 2, wherein the length of eachrespective one of said second line and said third line in the directionof the width is specified to be equal, said second line and said thirdline are arranged symmetrically with regard to the plane formed by thecenter of said first line in the direction of the width, one terminal ofsaid first line being assumed as an input terminal for unbalanced-modesignals is connected to one terminal of said third line as said inputterminal for unbalanced-mode signals, the other terminal of said firstline and one terminal of said second line are connected to the groundsurface respectively, and the other terminal of said second line and theother terminal of said third line are assumed as output terminals forbalanced-mode signal, the impedance of said input terminal forunbalanced-mode signals and the impedance of said each output terminalfor balance signals being specified to satisfy the followingrelationship defined below:(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in) ×Z _(out))^(1/2) where Ca isthe capacitance (C) between said second line and the ground surface, Cacis the capacitance (C) between said second line and said third line, sois the permittivity in the vacuum, ε_(r) is the relative permittivity,Z_(air) is the characteristic impedance (Ω) in the vacuum, Z_(in) is theimpedance (Ω) of said input terminal for unbalanced-mode signals, andZ_(out) is the impedance (Ω) of said output terminal for balanced-modesignals.
 6. The balun as defined in claim 3, wherein the length of eachrespective one of said second line and said third line in the directionof the width is specified to be equal, said second line and said thirdline are arranged symmetrically with regard to the plane formed by thecenter of said first line in the direction of the width, one terminal ofsaid first line being assumed as an input terminal for unbalanced-modesignals is connected to one terminal of said third line as said inputterminal for unbalanced-mode signals, the other terminal of said firstline and one terminal of said second line are connected to the groundsurface respectively, and the other terminal of said second line and theother terminal of said third line are assumed as output terminals forbalanced-mode signal, the impedance of said input terminal forunbalanced-mode signals and the impedance of said each output terminalfor balance signals being specified to satisfy the followingrelationship defined below:(Ca+Cac)/ε₀=ε_(r) ^(1/2) ×Z _(air)/(Z _(in) ×Z _(out))^(1/2) where Ca isthe capacitance (C) between said second line and the ground surface, Cacis the capacitance (C) between said second line and said third line, sois the permittivity in the vacuum, ε_(r) is the relative permittivity,Z_(air) is the characteristic impedance (Ω) in the vacuum, Z_(in) is theimpedance (Ω) of said input terminal for unbalanced-mode signals, andZ_(out) is the impedance (Ω) of said output terminal for balanced-modesignals.